Fix file header for ELPA compatibility

This commit makes `sf.el` installable with `package.el`.

- Add header line
- Fix footer line
- Add Commentary section
- Add Code section
- Add licence
- Move require to beginning of Code section

For information about this fix, See following GNU Emacs Manual:

- [Simple-Packages](http://www.gnu.org/software/emacs/manual/html_node/elisp/Simple-Packages.html)
- [Library-Headers](http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html)